This proposal describes experiments designed to investigate the regulation and enzymology of one-carbon metabolism mediated by folate coenzymes. Specifically, the regulation of C1-THF synthase, the central enzyme in folate interconversions, will be investigated in the rat as a mammalian model. Regulation of the enzyme will be examined under four experimental paradigms, chosen because they all involve perturbations of the one-carbon pathways. These paradigms include nutrition (vitamin B12 and folate deficiencies, methanol toxicity, altered thyroid function, and fructose-induced hyperuricemia), development, organ regeneration, and neoplasia. C1-THF synthase specific activities, concentration, and mRNA levels will be determined in an effort to characterize not only the spectrum of regulatory responses, but also the level and potential mechanism(s) of those responses. The enzyme will be purified and characterized from rat liver and a cDNA clone will be isolated and sequenced to generate structural information on the enzyme. The final phase of the project is directed at relating the findings in the rat model to human disease states by purifying and characterizing the enzyme from human tissue and obtaining a cDNA clone for the human enzyme to aid in structural analysis of that protein. It is hoped that these studies will add to our understanding of the regulation of this central area of metabolism and allow more rational basis for the treatment and maintenance of human disorders related to one-carbon metabolism.